Field of the Invention
The invention concerns a method for determining basic shim settings of a magnetic resonance scanner, and a computer, a non-transitory, data storage medium encoded with programming instructions, and a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
In a magnetic resonance apparatus, also referred to as a magnetic resonance tomography system, the body of a person to be examined, in particular a patient, is usually exposed by a basic field magnet to a relatively high basic magnetic field, for example of 1.5 or 3 or 7 Tesla. In addition gradient pulses are applied by a gradient coil unit. Radio-frequency pulses (excitation pulses) are then radiated by suitable antenna devices via a radio-frequency antenna unit, which cause the nuclear spins of specific atoms in the subject to be resonantly excited so as to be flipped by a defined flip angle in relation to the magnetic field lines of the basic magnetic field. During the relaxation of the nuclear spins, radio-frequency signals, so-called magnetic resonance signals, are emitted, which are received by suitable radio-frequency antennas and are then further processed. The desired image data can be reconstructed from the raw data acquired in this manner.
For a specific measurement (raw data acquisition), a specific magnetic resonance sequence, known as a pulse sequence, is executed, which is a sequence of radio-frequency pulses, especially excitation pulses and refocusing pulses, as well as gradient pulses matched thereto, emitted on different gradient axes along different spatial directions. Matching these pulses in time, readout windows are set, which predefine the periods of time in which the induced magnetic resonance signals are detected.
During magnetic resonance imaging with a magnetic resonance scanner, the homogeneity of a basic magnetic field in an examination volume of the scanner is of great significance. Even small deviations in the homogeneity can result in large deviations in the frequency distribution of the nuclear spins, so that the magnetic resonance image data recorded is of lower quality.
In order to improve the homogeneity in the examination volume, a magnetic resonance scanner typically has a shim unit. This shim unit usually has multiple shim elements. If a magnetic resonance scanner is installed at its intended location, then fields present in the environment can impair the homogeneity of the basic magnetic field, especially around an isocenter of the magnetic resonance scanner. Therefore the shim unit is set during installation and commissioning of a magnetic resonance scanner, frequently in conjunction with measurements, so that an optimum homogeneity is established. Thus basic shim settings can be determined for the installation and commissioning of the magnetic resonance scanner. The basic shim settings then include settings of the shim unit. Typically the basic shim settings are defined in such cases during the installation and commissioning of the magnetic resonance scanner.